The present invention relates to an (electro-)conductive roller, and a process cartridge and an image forming apparatus including the conductive roller. More specifically, the present invention relates to a conductive roller used as a charging member for charging a surface of an electrophotographic photosensitive member as a member to be charged to a prescribed potential by the application of voltage, and a process cartridge and an image forming apparatus including such a conductive roller.
As a contact charging member for charging an electrophotographic photosensitive member in an image forming apparatus, there has been conventionally used a charging roller comprising an electroconductive support (core metal) coated successively with an electroconductive elastic layer and a resistance layer. As for the charging scheme, an AC+DC-charging scheme of applying a superposition of AC voltage and DC voltage to the core metal has been principally adopted. In this case, the AC voltage used for realizing charging uniformity has been set to have a peak-to-peak voltage Vpp which is at least two times a charging initiation voltage under DC voltage application. In recent years, a DC charging scheme of applying only a DC voltage to a contact charging member has been commercialized.
A conventional charging roller used as a contact charging member has, more specifically, a structure including an electroconductive support (core metal) coated with an electroconductive elastic layer for providing uniform abutment against an electrophotographic photosensitive member as a member to be charged and further with a resistance layer for resistivity adjustment, wear resistance and prevention of sticking onto the photosensitive member. A structure is also known including optionally an intermediate layer for resistivity adjustment or prevention of exudation of a low-molecular weight component from the elastic layer.
As the electroconductive support, a metal bar of iron, stainless steel, etc., has been used.
The electroconductive elastic layer has been formed as a layer of elastomers or rubber, such as ethylene-propylene rubber (EPDM), butadiene rubber (BR), isoprene roller (IR), styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR) or silicone rubber, containing electroconductive inorganic particles, such as electroconductive carbon black, dispersed therein for resistivity adjustment.
The resistance layer has been formed of a resin, such as polyamide resin or polyurethane resin, containing electroconductive inorganic particles of, e.g., carbon black, titanium oxide or tin oxide, dispersed therein for resistivity adjustment.
However, charging performance, inclusive of charging uniformity of the charging member, is liable to be affected by good or poor dispersion of the electroconductive particles in the electroconductive elastic layer in not a few cases. For example, poor dispersion of electroconductive carbon black in the rubber has resulted in resistance irregularity of the charging member, leading to the occurrence of density irregularity or black spots corresponding to rotation periods of the charging member in halftone images or solid white images. The influence of such dispersion irregularity has occurred more frequently in the case of the DC charging scheme applying only a DC voltage.
For the above reason, instead of the electroconductive inorganic particles, such as carbon black, it has been tried to use ionic conductive agents, such as alkaline metal salts of perchloric acid and long-chain alkylsulfonic acids, and tetraalkyl-quaternary ammonium salts.
However, though the use of such a known ionic conductive agent provides an improved dispersibility of the conductive agent, a larger fluctuation of resistivity is liable to be caused by a change of environmental condition. For example, the charging member is liable to show a higher resistivity in a low humidity environment and a lower resistivity in a high humidity environment. Moreover, in the case of using such a known ionic conductive agent, the ionic conductive agent is liable to bleed out of the electroconductive elastic layer with time, thus changing the resistivity of the charging member. Further, the ionic conductive agent having bled out of the electroconductive elastic layer migrates to the surface of the resistance layer, thereby causing cracks in the surface of the electroconductive photosensitive member contacting the charging member and sticking with the photosensitive member. Further, because of the ionic conductive agent having migrated to the surface of the resistance layer, the developer is liable to be attached to the charging member surface, thus causing a periodical density irregularity in the resultant images corresponding to the rotation period of the charging member.
Further, in the case of using a charging member including an electroconductive elastic layer containing an alkaline metal perchlorate as an ionic conductive agent according to the DC charging scheme for image formation, halftone images formed in a low temperature/low humidity environment are liable to be accompanied with minute black spots, black streaks or lateral white streaks.
In view of the above-mentioned problems, an object of the present invention is to provide a conductive roller as a photosensitive member capable of stably providing high-quality images for a long period.
Another object of the present invention is to provide a process cartridge and an image forming apparatus including such a conductive roller.
According to the present invention, there is provided a conductive roller, comprising:
an electroconductive support, an electroconductive elastic layer coating the support and a resistance layer coating the elastic layer; wherein
the elastic layer comprises at least one species of rubber selected from the group consisting of acrylonitrile-butadiene rubber, epichlorohydrin rubber and chloroprene rubber, an ether oxygen-containing alkyl phthalate derivative, a quaternary ammonium perchlorate compound and a fatty oil, and
the ether oxygen-containing alkyl phthalate derivative, quaternary ammonium perchlorate compound and fatty oil are contained in a total amount of 0.1-20 wt. parts per 100 wt. parts of the rubber.
The present invention further provides:
a process cartridge, comprising an electrophotographic photosensitive member, and the above-mentioned conductive roller disposed as a charging member in contact with the electrophotographic photosensitive member; and
an electrophotographic apparatus, comprising an electrophotographic photosensitive member, the above-mentioned conductive roller being disposed as a charging member in contact with the electrophotographic photosensitive member, an exposure means and a transfer means.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.